Liquid-crystalline medium

ABSTRACT

The invention relates to a liquid-crystalline medium based on a mixture of polar compounds of positive dielectric anisotropy, containing at least one compound of the general formula I                    
     wherein R 1 -R 4  may each, independently of one another, be an alkyl or alkenyl radical having up to 12 carbon atoms which is unsubstituted, monosubstituted by CN or CF 3  or at least monosubstituted by halogen, where one or more CH 2  groups in these radicals may also, in each case independently of one another, be replaced by —O—, —S—,                    
     —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that O atoms are not linked directly to one another, and —A— is 1,4-phenylene, in which, in addition, one or two CH groups may be replaced by N, 2,3-difluoro-1,4-phenylene, 2-fluoro-1,4-phenylene, 3-fluoro-1,4-phenylene or a single bond.

The present invention relates to a liquid-crystalline medium, and to theuse thereof for electro-optical purposes, and to displays containingthis medium.

Liquid-crystals are used principally as dielectrics in display devices,since the optical properties of such substances can be modified by anapplied voltage. Electro-optical devices based on liquid crystals areextremely well known to the person skilled in the art and can be basedon various effects. Examples of such devices are cells having dynamicscattering, DAP (deformation of aligned phases) cells, guest/host cells,TN cells having a twisted nematic structure, STN (supertwisted nematic)cells, SBE (superbirefringence effect) cells and OMI (optical modeinterference) cells. The commonest display devices are based on theSchadt-Helfrich effect and have a twisted nematic structure.

The liquid-crystal materials must have good chemical and thermalstability and good stability to electric fields and electromagneticradiation. Furthermore, the liquid-crystal materials should have lowviscosity and produce short addressing times, low threshold voltages andhigh contrast in the cells.

They should furthermore have a suitable mesophase, for example a nematicor cholesteric mesophase for the abovementioned cells, in the usualoperating temperatures, i.e. in the broadest possible range above andbelow room temperature. Since liquid crystals are generally used asmixtures of a plurality of components, it is important that thecomponents are readily miscible with one another. Further properties,such as the electrical conductivity, the dielectric anisotropy and theoptical anisotropy, must satisfy arious requirements depending on thecell type and area of application. For example, materials for cellshaving a twisted nematic structure should have positive dielectricanisotropy and low electrical conductivity.

For example, media having large positive dielectric anisotropy, broadnematic phases, relatively low birefringence, very high specificresistance, good UV and temperature stability and low vapour pressureare desired for matrix liquid-crystal displays containing integratednon-linear elements for switching individual pixels (MLC displays).

Matrix liquid-crystal displays of this type are known. Non-linearelements which can be used for individual switching of the individualpixels are, for example, active elements (i.e. transistors). Referenceis then made to an “active matrix”, where a distinction can be madebetween two types:

1. MOS (metal oxide semiconductor) or other diodes on a silicon wafer assubstrate.

2. Thin-film transistors (TFTs) on a glass plate as substrate.

The use of monocrystalline silicon as substrate material restricts thedisplay size, since even modular assembly of various part-displaysresults in problems at the joints.

In the case of more-promising type 2, which is preferred, theelectro-optical effect used is usually the TN effect. A distinction ismade between two technologies: TFTs comprising compound semiconductors,such as, for example, CdSe or TFTs based on polycrystalline or amorphoussilicon. The latter technology is being worked on intensively worldwide.

The TFT matrix is applied to the inside of one glass plate of thedisplay, while the other glass plate carries the transparentcounterelectrode on its inside. In contrast to the size of the pixelelectrode, the TFT is very small and has virtually no interfering effecton the image. This technology can also be expanded to fullycolour-compatible displays, in which a mosaic of red, green and bluefilters is arranged in such a way that a filter element is opposite eachswitchable pixel.

The TFT displays usually operate as TN cells with crossed polarizers intransmission and are illuminated from the back.

The term MLC displays here covers any matrix display containingintegrated non-linear elements, i.e., besides the active matrix, alsodisplays containing passive elements, such as varistors or diodes(MIM=metal-insulator-metal).

MLC displays of this type are particularly suitable for TV applications(for example pocket TVs) or for high-information displays for computerapplications (laptops) and in automobile or aircraft construction.Besides problems regarding the angle dependence of the contrast and theresponse times, difficulties also arise in MLC displays due to theinsufficiently high specific resistance of the liquid-crystal mixtures[TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K.,TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p.141 ff, Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Designof Thin Film Transistors for Matrix Addressing of Television LiquidCrystal Displays, p. 145 ff, Paris]. With decreasing resistance, thecontrast of a MLC display worsens, and the problem of after-imageelimination can occur. Since the specific resistance of theliquid-crystal mixture generally drops over the life of an MLC displayowing to interaction with the interior surfaces of the display, a high(initial) resistance is very important in order to obtain acceptableservice lives. In particular in the case of low-volt mixtures, it washitherto impossible to achieve very high specific resistance values. Itis furthermore important that the specific resistance exhibits thesmallest possible increase with increasing temperature and after heatingand/or UV exposure. The low-temperature properties of the mixtures ofthe prior art are also particularly disadvantageous. The demands arethat no crystallization and/or smectic phases occur, even at lowtemperatures, and the temperature dependence of the viscosity is as lowas possible. The MLC displays from the prior art thus do not meettoday's requirements.

Besides liquid-crystal displays which use back illumination, i.e. areoperative transmissively and optionally transflectively, there is alsoparticular interest in reflective liquid-crystal displays. Thesereflective liquid-crystal displays use the ambient light for informationdisplay. They thus consume significantly less energy thanback-illuminated liquid-crystal displays of corresponding size andresolution. Since the TN effect is characterized by very good contrast,reflective displays of this type are readily legible even under brightambient conditions. This is already known of simple reflective TNdisplays, as used, for example, in wristwatches and pocket calculators.However, the principle can also be applied to high-quality,higher-resolution active matrix-addressed displays, such as, forexample, TFT displays. Here, as is already the case in the generallyconventional transmissive TFT-TN displays, the use of liquid crystals oflow birefringence (An) is necessary in order to achieve lowoptical-retardation (d An). This low optical retardation results in alow viewing-angle dependence of the contrast, which is usuallyacceptable (cf. DE 30 22 818). In reflective displays, the use of liquidcrystals of low birefringence is much more important than intransmissive displays, since in reflective displays, the effective layerthickness, through which the light passes, is approximately twice aslarge as in transmissive displays of the same layer thickness.

Besides the lower power consumption (no back-illumination necessary),other advantages of reflective displays over transmissive displays arethe space saving, which results in a very low installation depth, andthe reduction in problems caused by temperature gradients due to variousheating by the back-illumination. Voltage connection is thus simplersince there is no backlight supply.

There thus continues to be a great demand for MLC displays, inparticular reflective MLC displays having very high specific resistanceat the same time as a large working-temperature range, short responsetimes even at low temperatures and low threshold voltage which do nothave these disadvantages, or only do so to a reduced extent.

In TN (Schadt-Helfrich) cells, media are desired which facilitate thefollowing advantages in the cells:

expanded nematic phase range (in particular down to low temperatures)

rapid switching at extremely low temperatures (outdoor use, automobile,avionics)

increased resistance to UV radiation (longer life and simplifies thepanel bonding process)

lower threshold (addressing) voltage.

The media available from the prior art do not allow these advantages tobe achieved while simultaneously achieving the other parameters.

In the case of supertwisted (STN) cells, media are desired which enablegreater multiplexability and/or lower threshold voltages and/or broadernematic phase ranges (in particular at low temperatures). To this end, afurther increase in the available parameter latitude (clearing point,smectic-nematic transition or melting point, viscosity, dielectricparameters, elastic parameters) is urgently desired.

The invention has the object of providing media for these MLC, TN or STNdisplays, in particular for reflective MLC displays, which do not havethe abovementioned disadvantages or only do so to a reduced extent, andpreferably simultaneously have very high specific resistance values andlow threshold voltages and at the same time very low birefringencevalues.

It has now been found that this object can be achieved if mediaaccording to the invention are used in displays.

The invention thus relates to a liquid-crystalline medium based on amixture of polar compounds of positive dielectric anisotropy,characterized in that it comprises one or more compounds of generalformula I

in which

R¹-R⁴ are each, independently of one another, an alkyl or alkenylradical having up to 12 carbon atoms which is unsubstituted,monosubstituted by CN or CF₃ or at least monosubstituted by halogen,where one or more CH₂ groups in these radicals may also, in each caseindependently of one another, be replaced by

 —O—, —S—,

 —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that O atoms are notlinked directly to one another, and

—A— is 1,4-phenylene, in which, in addition, one or two CH groups may bereplaced by N, 2,3-difluoro-1,4-phenylene, 2-fluor-1,4-phenylene,3-fluoro-1,4-phenylene or a single bond.

The compounds of the formula I have a broad range of applications.Depending on the choice of substituents, these compounds can serve asbase materials of which liquid-crystalline media are predominantlycomposed; however, it is also possible to add compounds of the formula Ito liquid-crystalline base materials from other classes of compound inorder, for example, to modify the dielectric and/or, in particular, theoptical anisotropy of a dielectric of this type and/or to optimize itsthreshold voltage and/or its viscosity.

In the pure state, the compounds of the formula I are colourless andform liquid-crystalline mesophases in a temperature range which isfavourably located for electro-optical use. They are stable chemically,thermally and to light.

Particular preference is given to compounds of the formula I in which R¹to R⁴ are straight-chain alkyl radicals, furthermore straight-chainalkoxy radicals, having 1 to 7 carbon atoms.

If one of the radicals R¹ to R⁴ is an alkyl radical and/or an alkoxyradical, this can be straight-chain or branched.

R¹ to R⁴ are preferably straight-chain, have 2, 3, 4, 5, 6 or 7 carbonatoms and accordingly are preferably ethyl, propyl, butyl, pentyl,hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy or heptoxy,furthermore methyl, octyl, nonyl, decyl, undecyl, dodecyl, methoxy,octoxy, nonoxy, decoxy, undecoxy or dodecoxy.

Oxaalkyl is preferably straight-chain 2-oxapropyl (=methoxymethyl), 2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or 4-oxapentyl,2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl, 2-, 3-, 4-, 5-,6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl, or 2-, 3-, 4-,5-, 6-, 7-, 8- or 9-oxadecyl.

If R¹ to R⁴, independently of one another, are alkyl radicals in whichone CH₂ group has been replaced by —CH═CH—, this can be straight-chainor branched. They are preferably straight-chain and have 2 to 10 carbonatoms and are vinyl, 1E-alkenyl or 3E-alkenyl. Accordingly, they are inparticular vinyl, prop-1- or -2-enyl, but-1-, -2- or -3-enyl, pent-1-,-2-, -3- or -4-enyl, hex-1-, -2-, -3-, -4- or -5-enyl, hept-1-, -2-,-3-, -4-, -5- or -6-enyl, furthermore oct-1-, -2-, -3-, -4-, -5-, -6- or-7-enyl, non-1-, -2-, -3-, -4-, -5-, -6-, -7- or -8-enyl, or dec-1-,-2-, -3-, -4-, -5-, -6-, -7-, -8- or -9-enyl.

If R¹ to R⁴ are alkyl radicals in which one CH₂ group has been replacedby —O—and one has been replaced by —CO—, these are preferably adjacent.These thus contain an acyloxy group —CO—O— or an oxycarbonyl group—O—CO—. These are preferably straight-chain and have 2 to 6 carbonatoms. They are accordingly in particular acetoxy, propionyloxy,butyryloxy, pentanoyloxy, hexanoyloxy, acetoxymethyl,propionyloxymethyl, butyryloxymethyl, pentanoyloxymethyl,2-acetoxyethyl, 2-propionyloxyethyl, 2-butyryloxyethyl, 3-acetoxypropyl,3-propionyloxypropyl, 4-acetoxybutyl, methoxycarbonyl, ethoxycarbonyl,propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl,ethoxycarbonylmethyl, propoxycarbonylmethyl, butoxycarbonylmethyl,2-(methoxycarbonyl)ethyl, 2-(ethoxycarbonyl)ethyl,2-(propoxycarbonyl)ethyl, 3-(methoxycarbonyl)propyl,3-(ethoxycarbonyl)propyl or 4-(methoxycarbonyl)butyl.

If R¹ to R⁴ are alkyl radicals in which one CH₂ group has been replacedby unsubstituted or substituted —CH═CH— and an adjacent CH₂ group hasbeen replaced by CO or CO—O or O—CO, these can be straight-chain orbranched. They are preferably straight-chain and have 4 to 12 carbonatoms. Accordingly, they are in particular acryloyloxymethyl,2-acryloyloxyethyl, 3-acryloyloxypropyl, 4-acryloyloxybutyl,5-acryloyloxypentyl, 6-acryloyloxyhexyl, 7-acryloyloxyheptyl,8-acryloyloxyoctyl, 9-acryloyloxynonyl, 10-acryloyloxydecyl,methacryloyloxymethyl, 2-methacryloyloxyethyl, 3-methacryloyloxypropyl,4-methacryloyloxybutyl, 5-methacryloyloxypentyl, 6-methacryloyloxyhexyl,7-methacryloyloxyheptyl, 8-methacryloyloxyoctyl or9-methacryloyloxynonyl.

If R¹ to R⁴, independently of one another, are an alkyl or alkenylradical which is monosubstituted by CN or CF₃, this radical ispreferably straight-chain. The substitution by CN or CF₃ is in anydesired position.

If R¹ to R⁴, independently of one another, are alkyl or alkenyl radicalwhich is at least monosubstituted by halogen, these radicals arepreferably straight-chain and halogen is preferably F or Cl. In the caseof polysubstitution, halogen is preferably F. The resultant radicalsalso include perfluorinated radicals. In the case of monosubstitution,the fluorine or chlorine substituent can be in any desired position, butis preferably in the ω-position.

Compounds of the formula I which contain wing groups R¹ to R⁴ which aresuitable for polymerization reactions are suitable for the preparationof the liquid-crystalline polymers.

Compounds of the formula I containing branched wing groups R¹ to R⁴ mayoccasionally be of importance owing to better solubility in theconventional liquid-crystalline base materials, but in particular aschiral dopants if they are optically active. Smectic compounds of thistype are suitable as components of ferro-electric materials.

Compounds of the formula I having S_(A) phases are suitable, forexample, for thermally addressed displays.

Branched groups generally contain not more than one chain branch.Preferred branched radicals R¹ to R⁴ are isopropyl, 2-butyl(=1-methylpropyl), isobutyl (=2-methylpropyl), 2-methylbutyl, isopentyl(=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl,2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy,3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy,1-methylhexoxy or 1-methylheptoxy.

If one of the radicals R¹ to R⁴ is an alkyl radical in which two or moreCH₂ groups have been replaced by —O— and/or —CO—O—, this can bestraight-chain or branched. It is preferably branched and has 3 to 12carbon atoms. Accordingly, it is in particular biscarboxymethyl,2,2-biscarboxyethyl, 3,3-biscarboxypropyl, 4,4-biscarboxybutyl,5,5-biscarboxypentyl, 6,6-biscarboxyhexyl, 7,7-biscarboxyheptyl,8,8-biscarboxyoctyl, 9,9-biscarboxynonyl, 10,10-biscarboxydecyl,bis-(methoxycarbonyl)methyl, 2,2-bis(methoxycarbonyl)ethyl,3,3-bis(methoxycarbonyl)propyl, 4,4-bis(methoxycarbonyl)butyl,5,5-bis(methoxycarbonyl)pentyl, 6,6-bis(methoxycarbonyl)hexyl,7,7-bis(methoxycarbonyl)-heptyl, 8,8-bis(methoxycarbonyl)octyl,bis(ethoxycarbonyl)methyl, 2,2-bis(ethoxycarbonyl)ethyl,3,3-bis-(ethoxycarbonyl)propyl, 4,4-bis(ethoxycarbonyl)butyl or5,5-bis(ethoxycarbonyl)hexyl.

Preferred smaller groups of compounds of the formula I are those of thesub-formulae I1 to I5:

In the sub-formulae I1 to I5,

r, s, t and u are each, independently of one another, 1-12,

v, w, x and y are each, independently of one another, from 0 to 10,

where v+w and x+y are <10.

Particular preference is given to the compounds of the formulae I1 andI3.

The compounds of the formula I are prepared by methods known per se, asdescribed in the literature (for example in the standard works, such asHouben-Weyl, Methoden der Organischen Chemie [Methods of OrganicChemistry], Georg-Thieme-Verlag, Stuttgart), to be precise underreaction conditions which are known and suitable for said reactions. Usecan also be made here of variants which are known per se, but are notmentioned here in greater detail.

The compounds according to the invention can be prepared, for example,as follows:

EXAMPLE 1

EXAMPLE 2

The invention also relates to electro-optical displays (in particularSTN or MLC displays having two plane-parallel outer plates, which,together with a frame, form a cell, integrated non-linear elements forswitching individual pixels on the outer plates, and a nematicliquid-crystal mixture of positive dielectric anisotropy and highspecific resistance which is located in the cell) which comprise mediaof this type, and to the use of these media for electro-opticalpurposes.

The liquid-crystal mixtures according to the invention allow asignificant increase in the parameter latitude which is available.

The achievable combinations of rotational viscosity γ₁, clearing point,flow viscosity at low temperature, thermal and UV stability anddielectric and optical anisotropy are far superior to the knownmaterials from the prior art. In particular, the achievable combinationof low birefringence and at the same time high clearing point throughthe use of one or more compounds of the formula I enables thepreparation of mixtures comprising no or only few low An compounds,which enables the proportion by weight of the polar liquid-crystallinecompounds in the mixture to be drastically increased. This has theindirect consequence that the threshold voltage is reduced.

The requirement for a high clearing point, nematic phase at lowtemperature and low birefringence (An) and simultaneously a lowthreshold voltage has hitherto only been achieved inadequately. Althoughliquid-crystal mixtures such as, for example, MLC-6476 and MLC-6625(Merck KGaA, Darmstadt, Germany) have comparable clearing points andlow-temperature stabilities, they both have, however, much higher Anvalues of about 0.075 and much higher threshold voltages of about ≧1.7 Vor more.

While retaining the nematic phase down to −20° C., preferably down to−30° C., particularly preferably down to −40° C., and clearing pointsabove 80° C., preferably above 90° C., particularly preferably above100° C., the liquid-crystal mixtures according to the inventionsimultaneously allow birefringence values of ≦0.07, preferably ≦0.065,very particularly preferably ≦0.0635, and a low threshold voltage,allowing excellent STN and MLC displays, in particular reflective MLCdisplays, to be achieved. In particular, the mixtures are characterizedby low operating voltages.

The TN thresholds are below 2.0 V, preferably below 1.8 V. Particularpreference is given to mixtures according to the invention which have aclearing point of 80° C. or above and

a threshold voltage of ≦1.90 V and a Δn of ≦0.0625, or

a threshold voltage of ≦1.80 V and a Δn of ≦0.0615, or

a threshold voltage of ≦1.80 V and a Δn of ≦0.060.

It is evident to the person skilled in the art that a suitable choice ofthe components of the mixtures according to the invention also allowshigher clearing points (for example above 110° C.) to be achieved at thesame time as lower dielectric anisotropy values and thus higherthreshold voltages, or lower clearing points to be achieved at the sametime as higher dielectric anisotropy values (for example >12) and thuslower threshold voltages (for example <1.1 V) while retaining the otheradvantageous properties.

Likewise, mixtures of higher Δε and thus lower thresholds can also beobtained at viscosities which are increased correspondingly little. TheMLC displays according to the invention preferably operate at the firstGooch and Tarry transmission minimum [C. H. Gooch and H. A. Tarry,Electron. Lett. 10, 2-4, 1974; C. H. Gooch and H. A. Tarry, Appl. Phys.,Vol. 8, 1575-1584, 1975], where, besides particularly favourableelectro-optical properties, such as, for example, high steepness of thecharacteristic line and low angle dependence of the contrast (GermanPatent 30 22 818), a lower dielectric anisotropy is sufficient at thesame threshold voltage as in an analogous display at the second minimum.Thus, significantly higher specific resistance values can be achievedusing the mixtures according to the invention at the first minimum thanin the case of mixtures comprising cyano compounds.

Through a suitable choice of the individual components and theirproportions by weight, the person skilled in the art can set thebirefringence necessary for a specified layer thickness of the MLCdisplay using simple routine methods. The requirements of reflective MLCdisplays are described, for example, in Digest of Technical Papers, SIDSymposium 1998.

The rotational viscosity γ₁ is preferably <140 mpa·s, particularlypreferably <120 mPa·s. The nematic phase range is preferably at least90°, in particular at least 100°. This range preferably extends at leastfrom −20° to +80°.

Measurements of the capacity holding ratio, also known as the voltageholding ratio (HR) [S. Matsumoto et al., Liquid Crystals 5, 1320 (1989);K. Niwa et al., Proc. SID Conference, San Francisco, June 1984, p. 304(1984); G. Weber et al., Liquid Crystals 5, 1381 (1989)] have shown thatmixtures according to the invention comprising compounds of the formulaI have a significantly smaller drop in HR with increasing temperaturethan analogous mixtures in which the compounds of the formula I havebeen replaced by cyanophenylcyclohexanes of the formula

or esters of the formula

The UV stability of the mixtures according to the invention is alsoconsiderably better, i.e. they exhibit a significantly smaller drop inHR on UV exposure.

The media according to the invention are preferably based on a plurality(preferably two, three or more) of compounds of the formula I, i.e. theproportion of these compounds is 5-95%, preferably 10-60%, particularlypreferably in the range 15-50%.

The individual compounds of the formulae I to XVI and their sub-formulaewhich can be used in the media according to the invention are eitherknown or can be prepared analogously to the known compounds.

Preferred embodiments are indicated below:

The medium comprises compounds of the formula I in which at least one ofthe radicals R¹ to R⁴ is preferably ethyl, propyl, butyl, pentyl, hexyland/or heptyl. Compounds of the formula I having short side chains R¹ toR⁴ have a positive effect on the elastic constants, in particular K₁,and result in mixtures having particularly low threshold voltages.

The medium additionally comprises one or more compounds selected fromthe group consisting of the general formulae II to IX:

in which the individual radicals have the following meanings:

R⁰: n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each case having up to9 carbon atoms,

X⁰: F, Cl, halogenated alkyl, halogenated alkenyl or halogenated alkoxyhaving 1 to 6 carbon atoms,

Z⁰: —C₂H₄—, —CH═CH—, —C₂F₄—, —OCF₂—, —CF₂O—

Y¹ to Y⁴: each, independently of one another, H or F

r: 0 or 1.

The compound of the formula IV is preferably

The medium additionally comprises one or more compounds of the formula

in which

and L¹ is H or F.

The medium additionally comprises one or more compounds of the formulaeRI, RII and/or RIII:

in which R⁰ is as defined above and is preferably straight-chain alkylhaving 1-6 carbon atoms, and Alkenyl and Alkenyl* are each,independently of one another, preferably vinyl, 1E-alkenyl, 3E-alkenylor 4-alkenyl having up to 9 carbon atoms. Alkyl and Alkyl* arestraight-chain alkyl having 1-6 carbon atoms.

The term “Alkyl” or “Alkyl*” preferably covers straight-chain andbranched alkyl groups having 1-7 carbon atoms, in particular thestraight-chain groups methyl, ethyl, propyl, butyl, pentyl, hexyl andheptyl. Groups having 2-5 carbon atoms are generally preferred.

The term “Alkenyl” or “Alkenyl*” preferably covers straight-chain andbranched alkenyl groups having 2-7 carbon atoms, in particular thestraight-chain groups. Particularly preferred alkenyl groups are C₂-C₇-1E-alkenyl, C₄-C₇ -3E-alkenyl, C₅-C₇ -4-alkenyl, C₆-C₇ -5-alkenyl andC₇ -6-alkenyl, in particular C₂-C₇ -1E-alkenyl, C₄-C₇ -3E-alkenyl andC₅-C₇ -4-alkenyl. Examples of preferred alkenyl groups are vinyl,1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl,3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl,4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Alkenylgroups having up to 5 carbon atoms are generally preferred.

The medium additionally comprises one or more compounds selected fromthe group consisting of the general formulae X to XV:

in which R⁰, X⁰, Y¹ and Y² are each, independently of one another, asdefined above. X⁰ is preferably F, Cl, CF₃, OCF₃, OCHF₂, OCH═CF₂,OCF═CF₂ or OCF═CHF.

The proportion of compounds of the formulae I to IX in the mixture as awhole is at least 50% by weight;

The proportion of compounds of the formula I in the mixture as a wholeis from 5 to 50% by weight, preferably from 5 to 30% by weight;

The proportion of compounds of the formulae II to IX in the mixture as awhole is from 20 to 80% by weight;

The medium comprises compounds of the formulae II, III, IV, V, VI and/orVII;

R⁰ is straight-chain alkyl or alkenyl having 2 to 7 carbon atoms;

The medium essentially consists of compounds of the formulae I to IX;

The medium comprises further compounds, preferably selected from thefollowing group consisting of the general formulae XVII to XXII;

in which R⁰, X⁰ and X^(0′) are as defined above, and the 1,4-phenylenerings may be substituted by CN, chlorine or fluorine. The 1,4-phenylenerings are preferably mono- or polysubstituted by fluorine atoms.

The term “Fluoroalkyl” preferably covers straight-chain groups having aterminal fluorine, i.e. fluoromethyl, 2-fluoroethyl, 3-fluoropropyl,4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and 7-fluoroheptyl.However, other positions of the fluorine are not excluded.

The term “Oxaalkyl” preferably covers straight-chain radicals of theformula C_(n)H_(2n+1)—O—(CH₂)_(m), in which n and m are each,independently of one another, from 1 to 6. n is preferably 1 and m ispreferably from 1 to 6.

Through suitable choice of the meanings of R⁰, X⁰ and X^(0′), theaddressing times, the threshold voltage, the steepness of thetransmission characteristic lines, etc., can be modified in the desiredmanner. For example, 1E-alkenyl radicals, 3E-alkenyl radicals,2E-alkenyloxy radicals and the like generally result in short addressingtimes, improved nematic tendencies and a higher ratio of the elasticconstants k₃₃ (bend) and k₁₁ (splay) compared with alkyl or alkoxyradicals. 4-Alkenyl radicals, 3-alkenyl radicals and the like generallygive lower threshold voltages and smaller values of k₃₃/k₁₁ comparedwith alkyl and alkoxy radicals.

A —CH₂CH₂— group in a liquid-crystalline compound generally results inhigher values of k₃₃/k₁₁ compared with a single covalent bond. Highervalues of k₃₃/k₁₁ facilitate, for example, flatter transmissioncharacteristic lines in TN cells with a 90° twist (in order to achievegrey shades) and steeper transmission characteristic lines in STN, SBEand OMI cells (higher multiplexability), and vice versa.

The I: (II+III+IV+V+VI+VII+VIII+IX) weight ratio is preferably 1:10 to10:1;

The medium essentially consists of compounds selected from the groupconsisting of the general formulae I to XXII.

It has been found that even a relatively small proportion of compoundsof the formula I mixed with conventional liquid-crystal materials, butin particular with one or more compounds of the formulae II to IXresults in low birefringence values, where broad nematic phases with lowsmectic-nematic transition temperatures are simultaneously observed,improving the storage stability. The compounds of the formulae I to IXare colourless, stable and readily miscible with one another and withother liquid-crystalline materials.

The optimum mixing ratio of the compounds of the formulae I andII+III+IV+V+VI+VII+VIII+IX depends substantially on the desiredproperties, on the choice of the components of the formulae I, II, III,IV, V, VI, VII, VII and/or IX, and on the choice of any other componentswhich may be present. Suitable mixing ratios within the range givenabove can easily be determined from case to case.

The total amount of compounds of the formulae I to XVI in the mixturesaccording to the invention is not crucial. The mixtures can thereforecomprise one or more further components in order to optimize variousproperties. However, the observed effect on the addressing times and thethreshold voltage is generally greater the higher the totalconcentration of compounds of the formulae I to XVI.

In a particularly preferred embodiment, the media according to theinvention comprise compounds of :the formulae II to IX (preferably IIand/or III) in which X⁰ is OCF₃, OCHF₂, F, OCH═CF₂, OCF═CF₂ orOCF₂—CF₂H. A favourable synergistic effect with the compounds of theformula I results in particularly advantageous properties.

The construction of the MLC display according to the invention frompolarizers, electrode base plates and surface-treated electrodescorresponds to the conventional construction for displays of this type.

The term “conventional construction” is broadly drawn here and alsocovers all derivatives and modifications of the MLC display, inparticular including matrix display elements based on poly-Si TFT orMIM.

A significant difference between the displays according to the inventionand the conventional displays based on the twisted nematic cellconsists, however, in the choice of the liquid-crystal parameters of theliquid-crystal layer.

The liquid-crystal mixtures which can be used in accordance with theinvention are prepared in a manner conventional per se. In general, thedesired amount of the components used in a lesser amount is dissolved inthe components making up the principal constituent, expediently atelevated temperature. It is also possible to mix solutions of thecomponents in an organic solvent, for example in acetone, chloroform ormethanol, and to remove the solvent again after thorough mixing, forexample by distillation.

The dielectrics may also comprise further additives known to the personskilled in the art and described in the literature. For example, 0-15%of pleochroic dyes and/or chiral dopants can be added. The additives areeach employed in concentrations of from 0.01 to 6%, preferably from 0.1to 3%. However, the concentration data for the other constituents of theliquid-crystal mixtures, i.e. of the liquid-crystalline or mesogeniccompounds, are given without taking into account the concentration ofthese additives.

C denotes a crystalline phase, S a smectic phase, S_(C) a smectic Cphase, N a nematic phase and I the isotropic phase.

V₁₀ denotes the voltage for 10% transmission (viewing directionperpendicular to the plate surface). t_(on), denotes the switch-on timeand t_(off) the switch-off time at an operating voltage corresponding to2.5 times the value of V₁₀. Δn denotes the optical anisotropy, and n.the refractive index. Δε denotes the dielectric anisotropy (Δε=ε∥−ε⊥,where ε∥ denotes the dielectric constant parallel to the longitudinalaxis of the molecules, and ε⊥ denotes the dielectric constantperpendicular thereto). The electro-optical data were measured in a TNcell at the 1st minimum (i.e. at a d·Δn value of 0.5) at 20° C., unlessexpressly stated otherwise. The optical data were measured at 20° C.,unless expressly stated otherwise.

The physical properties of the liquid-crystal mixtures were determinedin accordance with “Physical Properties of Liquid Crystals”, Edition W.Becker 1997, Merck KGaA, unless explicitly stated otherwise.

In the present application and in the examples below, the structures ofthe liquid-crystal compounds are indicated by means of acronyms, thetransformation into chemical formulae taking place in accordance withTables A and B below. All radicals C_(n)H_(2n+1) and C_(m)H_(2m+1) arestraight-chain alkyl radicals having n and m carbon atoms respectively.Preferably, n and m denote 1-10. The coding in Table B is self-evident.

In Table A, only the acronym for the parent structure is given,followed, separated from the acronym for the parent structure by ahyphen, by a code for the substituents R^(1′), R^(2′), L^(1′) and L^(2′)

Code for R^(1′), R^(2′), L^(1′), L^(2′) R¹ R^(2′) L^(1′) L^(2′) nmC_(n)H_(2n+1) C_(m)H_(2m+1) H H nOm C_(n)H_(2n+1) OC_(m)H_(2m+1) H HnO.m OC_(n)H_(2n+1) C_(m)H_(2m+1) H H n C_(n)H_(2n+1) CN H H nN.FC_(n)H_(2n+1) CN H F nF C_(n)H_(2n+1) F H H nOF OC_(n)H_(2n+1) F H H nClC_(n)H_(2n+1) Cl H H nF.F C_(n)H_(2n+1) F H F nF.F.F C_(n)H_(2n+1) F F FnCF₃ C_(n)H_(2n+1) CF₃ H H nOCF₃ C_(n)H_(2n+1) OCF₃ H H nOCF₂C_(n)H_(2n+1) OCHF₂ H H nS C_(n)H_(2n+1) NCS H H rVsNC_(r)H_(2r+1)—CH═CH—C_(s)H_(2s)— CN H H rEsNC_(r)H_(2r+1)—O—C_(s)H_(2s)— CN H H nAm C_(n)H_(2n+1) COOC_(m)H_(2m+1) HH nOCCF₂.F.F C_(n)H_(2n+1) OCH₂CF₂H F F

Preferred mixture components of the liquid-crystalline mixturesaccording to the invention are shown in Tables A and B:

TABLE A

PYP

PYRP

BCH

CBC

CCH

CCP

CPTP

CEPTP

ECCP

CECP

EPCH

PCH

PTP

BECH

EBCH

CPC

B

FET-nF

CGG

CGU

CUP

CDU

TABLE B

BCH-n.Fm

GGP-n-Cl

Inm

CBC-nmF

ECCH-nm

CCH-n1EM

T-nFm

CGU-n-F

PGU-n-F

CCGU-n-F

CC-n-V

CCZU-n-F

CC-n-m-TT-CC-m-n

The examples below are intended to illustrate the invention withoutrepresenting a limitation. Above and below, percentages are percent byweight. All temperatures are given in degrees Celsius. m.p. denotesmelting point, cl.p. clearing point. Furthermore, C=crystalline state,N=nematic phase, S=smectic phase and I=isotropic phase. The numbersbetween these symbols are the transition temperatures. An denotes theoptical anisotropy (589 nm, 20° C.), and the flow viscosity (mm²/sec)and rotational viscosity γ₁ (mPa·s) were each determined at 20° C.

MIXTURE EXAMPLES EXAMPLE 1

CCH—3O1 10.0% S → N [° C.]: <−40° C. CC-5-V 17.0% Clearing point [° C.]:+89.5° C. CCP—2OCF₃ 3.0% Δn [589 nm, 20° C.]: +0.0598 CCP—2F.F.F 11.0%d.Δn [20° C.]: 0.50 μm CCP—3F.F.F 11.0% Twist: 90° CCP—5F.F.F 6.0%V_((10,0,20)) [V]: 1.77 CCP—2OCF₃.F 8.0% Tilt: 3.4° CCZU-2-F 5.0%CCZU-3-F 12.0% CCZU-5-F 5.0% CC-5-5-TT—CC-5-5 12.0%

EXAMPLE 2

CCH—3CF₃ 6.0% S → N [° C.]: <−40.0 CCH—5CF₃ 9.0% Clearing point [° C.]:+81.0 CCH-34 3.0% Δn [589 nm, 20° C.]: +0.0613 CC-5-V 6.0% Rotationalviscosity γ₁ CCP—2F.F.F 9.0% [m Pa · s; 20° C.]: 164 CCP—3F.F.F 10.0%d.Δn [20° C., μm]: 0.50 CCP—4F.F.F 8.0% Twist: 90° CCP—5F.F.F 5.0% V₁₀[V]: 1.55 CCP—4OCF₃ 2.0% CCP—2OCF₃.F 10.0% CCZU-2-F 5.0% CCZU-3-F 14.0%CCZU-5-F 5.0% CC-5-5-TT—CC-5-5 8.0%

EXAMPLE 3

CCH—3CF₃ 6.0% S → N [° C.]: <−40.0 CCH—5CF₃ 9.0% Clearing point [° C.]:+79.5 CCH-34 4.0% Δn [589 nm; 20° C.]: +0.0643 CC-5-V 5.0% Rotationalviscosity γ₁ CCP—2F.F.F 11.0% [m Pa · s; 20° C.]: 139 CCP—3F.F.F 12.0%d.Δn [20° C., μm]: 0.50 CCP—5F.F.F 6.0% Twist: 90° CCP—4OCF₃ 7.0% V₁₀[V]: 1.54 CCP—2OCF₃.F 8.0% CCP—2OCF₃ 3.0% CCZU-2-F 5.0% CCZU-3-F 15.0%CCZU-5-F 5.0% CC-5-5-TT—CC-5-5 4.0%

EXAMPLE 4

CCH—3CF₃ 7.0% S → N [° C.]: <−30.0 CCH—5CF₃ 7.0% Clearing point [° C.]:+80.5 CCH-34 5.0% Δn [589 nm; 20° C.]: 0.0653 CC-5-V 3.5% Rotationalviscosity γ₁ CCP—2F.F.F 12.0% [m Pa · s; 20° C., μm]: 141 CCP—3F.F.F12.0% d.Δn [20° C.]: 0.50 CCP—5F.F.F 6.0% Twist: 90° CCP—3OCF₃ 2.0% V₁₀[V]: 1.51 CCP—4OCF₃ 7.0% CCP—2OCF₃.F 10.0% CCZU-2-F 5.0% CCZU-3-F 15.0%CCZU-5-F 5.0% CC-5-5-TT—CC-5-5 3.5%

EXAMPLE 5

CCH—3CF₃ 9.0% S → N [° C.]: <−30.0 CCH—5CF₃ 7.0% Clearing point [° C.]:+80.0 CCH-34 5.0% Δn [589 nm; 20° C.]: 0.0652 CCP—2F.F.F 11.0%Rotational viscosity γ₁ CCP—3F.F.F 12.0% [m Pa · s; 20° C.]: 144CCP—5F.F.F 5.0% d.Δn [20° C., μm]: 0.50 CCP—2OCF₃ 4.0% Twist: 90°CCP—3OCF₃ 2.0% V₁₀ [V]: 1.49 CCP—4OCF₃ 7.0% CCP—2OCF₃.F 10.0% CCZU-2-F5.0% CCZU-3-F 15.0% CCZU-5-F 4.0% CC-5-5-TT—CC-5-5 4.0%

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

The entire disclosure of all applications, patents and publications,cited above, are hereby incorporated by reference.

This application claims priority under 35 USC Section 119 to GermanPatent Application 19857504.1, filed Dec. 14, 1998, which application isexpressly incorporated herein by reference.

Not intending to be limited by the specific examples and preferredembodiments discussed hereinbefore, but only by the claims which follow,the applicants claim:

What is claimed is:
 1. Liquid-crystalline medium based on a mixture ofpolar compounds of positive dielectric anisotropy, characterized in thatit comprises one or more compounds of the general formula I

in which R¹-R⁴ are each, independently of one another, an alkyl oralkenyl radical having up to 12 carbon atoms which is unsubstituted,monosubstituted by CN or CF₃ or at least monosubstituted by halogen,where one or more CH₂ groups in these radicals may also, in each caseindependently of one another, be replaced by —O—, —S—,

—CO—, —CO—O—, —O—CO— or —O—CO—O—in such a way that O atoms are notlinked directly to one another, and —A— is 1,4-phenylene, in which, inaddition, one or two CH groups may be replaced by N,2,3-difluoro-1,4-phenylene, 2-fluor-1,4-phenylene,3-fluoro-1,4-phenylene or a single bond.
 2. Medium according to claim 1,characterized in that R¹ to R⁴ are each, independently of one another, astraight-chain or branched alkyl or alkoxy radical having 1 to 7 carbonatoms.
 3. Medium according to claim 1, characterized in that itadditionally comprises one or more compounds selected from the groupconsisting of the general formulae II to IX:

in which the individual radicals have the following meanings: R⁰:n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each case having up to 9carbon atoms, X⁰: F, Cl, halogenated alkyl, alkenyl or alkoxy having 1to 6 carbon atoms, Z⁰: —C₂H₄—, —CH═CH—, —C₂F₄—, —OCF₂—, —CF₂O— —Y¹ toY⁴: each, independently of one another, H or F, r: 0 or
 1. 4. Mediumaccording to claim 3, characterized in that the proportion of compoundsof the formulae I to IX in the mixture as a whole is at least 50% byweight.
 5. Medium according to claim 1, characterized in that theproportion of compounds of the formula I in the mixture as a whole isfrom 5 to 50% by weight.
 6. Medium according to claim 3, characterizedin that the proportion of compounds of the formulae II to IX in themixture as a whole is from 20 to 80% by weight.
 7. Medium according toclaim 3, characterized in that X⁰ is F, OCHF₂ or OCF₃.
 8. Mediumaccording to claim 1, characterized in that it comprises one or morecompounds of the formula I and one or more compounds of the formula

in which

R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each case having upto 9 carbon atoms, and L¹ is H or F.
 9. A medium according claim 1,comprising one or more compounds selected from the general formulae X toXVI

in which R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each casehaving up to 9 carbon atoms, X⁰ is F, Cl, halogenated alkyl, alkenyl, oralkoxy having 1 to 6 carbon atoms, Y¹ and Y² are each, independently ofone another, H or F.
 10. An electro-optical display comprising twoplane-parallel outer plates, elements for switching individual pixels onthe plates, and a liquid-crystalline medium as claimed in claim
 1. 11. Areflective matrix liquid-crystal display comprising a liquid crystallinemedium as claimed in claim
 1. 12. Electro-optical liquid-crystal displaycontaining a liquid-crystalline medium according to claim
 1. 13. Aliquid crystalline medium as claimed in claim 1, wherein the medium hasa nematic phase below about −20° C., a clearing point above about 80°C., a birefringence of less than or equal to 0.07, and a thresholdvoltage of less than 2.0 V.
 14. A liquid crystalline medium as claimedin claim 1, wherein at least one of the radicals R¹ to R⁴ is ethyl,propyl, butyl, pentyl, hexyl, or heptyl.